Adjustable height fusion device

ABSTRACT

Method and apparatus for promoting a spinal fusion between neighboring vertebrae. Apparatus may be located within the intervertebral disc space and preferably includes a pair of engaging plates for contacting the vertebrae. An alignment device may be used to alter the vertical distance between the engaging plates to customize the apparatus to fit a given patient. In one embodiment, the alignment device includes a pair of struts having a predetermined height and extending between the engaging plates from an anterior end to a posterior end of the apparatus. In another embodiment, the alignment device includes a rotatable connector and cam pins for adjusting the distance between the engaging plates. The alignment device is preferably adapted to vary the distance between the engaging plates such that the height of the apparatus proximate the anterior end is greater than that proximate the posterior end whereby the natural lordosis of the spine is maintained after the apparatus is installed. The apparatus may further include a load-sharing member to allow stress to be imparted to bone in the vicinity of the apparatus to promote bone growth in accordance with Wolff&#39;s law.

This is a continuation of application Ser. No. 09/153,178 filed Sep. 15,1998, now U.S. Pat. No. 6,080,193 which is a divisional application ofSer. No. 08/847,172 filed May 1, 1997, which issued as U.S. Pat. No.6,045,579.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to methods and apparatus forpromoting an intervertebral fusion, and more particularly to anapparatus for insertion into a space between adjacent vertebrae tofacilitate an intervertebral fusion while maintaining a substantiallynatural lordosis of the human spine.

2. Description of the Related Art

Intervertebral discs that become degenerated due to various factors suchas trauma or aging typically have to be partially or fully removed.Removal of an intervertebral disc can destabilize the spine, making itnecessary to replace the vertebral disc to maintain the height of thespine and to fuse the spine. Spinal implants are often used to preventcollapse of the spine. U.S. Ser. No. 08/740,123 filed Oct. 24, 1996relates to methods and apparatus for facilitating a spinal fusion and isincorporated by reference as if fully set forth herein.

After an intervertebral disc is removed, an implant device is typicallyinserted between neighboring vertebrae to maintain normal disc spacingand restore spinal stability, thereby facilitating an intervertebralfusion. A conventional implant device disposed between neighboringvertebrae is depicted in FIGS. 1 and 2. The implant device contains apair of engaging elements 20 that typically contain threading 10 toengage the vertebrae. Prior to inserting the engaging elements, avertebral drill is typically inserted within the surgical wound to drillinto the cortical endplate and remove fibrous and nuclear material. Avertebral tap may then be used to cut threads into the ends of theneighboring vertebrae. The engaging elements tend to be relativelyinflexible and substantially undeflectable. The engaging elements aretypically packed with bone graft to facilitate a spinal fusion.

Conventional implant devices tend to not maintain the “lordosis” ornatural curvature of the lower lumbar spine. As shown in FIG. 1, theimplant device contains parallel engaging sides 12 and 13 to contactvertebra 15. It is typically required that the engaging sides beparallel to prevent the fusion cage from slipping from theintervertebral space. The parallel configuration of the fusion cagetends to alter the lordosis of the spine. Such a loss of lordosis mayresult in an increased risk to other intervertebral discs locatedadjacent to the fusion level that may degenerate due to the alteredforce transmission in the spine.

FIG. 2 depicts a front view of the engaging elements 20 of the implantdevice. The engaging elements are substantially cylindrical and theregion of contact between an engaging element and a vertebra is definedby arcuate portion 22. The cylindrical geometry of the engaging elementstends to provide a relatively small area of contact between the fusioncage and the vertebrae. The weight of the spine creates pressure on thevertebrae that is concentrated proximate the arcuate portions.Subsidence or deformation of the cortical layer of the vertebrae tendsto result.

U.S. Pat. No. 5,522,899 to Michelson relates to a spinal implant forplacement into the spinal disc space to stabilize the spine andparticipate in a vertebra to vertebra bony fusion. U. S. Pat. No.5,489,308 to Kuslich et al. relates to an implant for use in spinalstabilization that includes a cylindrical body having external threadingand radially disposed openings positioned to chip bone into an interiorportion of the body when the implant is installed. The above-mentionedpatents are incorporated by reference as if fully set forth herein.

The above-mentioned prior methods and systems inadequately address,among other things, the need to maintain the natural lordosis of thespine. It is therefore desirable that an improved spinal implant bederived for facilitating an intervertebral body fusion.

SUMMARY OF THE INVENTION

In accordance with the present invention, a spinal implant is providedthat largely eliminates or reduces the aforementioned disadvantages ofconventional implant devices. An embodiment of the invention relates toa fusion device for facilitating an interbody fusion between neighboringvertebrae of a human spine. The fusion device preferably includes a pairof sides or engaging plates for engaging the vertebrae and an alignmentdevice disposed between the engaging plates for separating the engagingplates to maintain the engaging plates in lordotic alignment. Thealignment device is preferably adapted to adjust the height between theengaging plates to customize the fusion device to a particular patient.The height of the fusion device preferably varies along the length ofthe device such that the height proximate an anterior end of the devicediffers from the height proximate a posterior end of the device.

The engaging plates are preferably substantially planar so as to inhibitsubsidence of the vertebrae. The engaging plates may contain protrusionsextending from their outer faces for enhancing an engagement between thevertebra and the engaging plate. The protrusions may be adapted toextend into the vertebra. The engaging plates preferably include aplurality of openings to allow bone growth to occur through the engagingplates. The openings in the face of the engaging plates preferably havea total area that is between about 60 percent and about 80 percent of atotal surface area of the face (including the area of the openings).

The fusion device may include a retaining plate proximate the posteriorend that serves as a backing against which bone graft may be packedbetween the engaging plates. The fusion device may also include aremovable end cap proximate the anterior end for maintaining bone graftbetween the engaging plates.

In an embodiment, the alignment device includes a first strut and asecond strut that each extend between the engaging plates to define theheight therebetween. The fusion device preferably includes a first sideand a second side opposite the first side. The first strut preferablyruns from the anterior end to the posterior end along a locationproximate the first side, and the second strut preferably runs from theanterior end to the posterior end along a location proximate the secondside. The engaging plates preferably include a pair of slots sized toreceive ends of the struts. The slots may have a substantiallydovetail-shaped cross-section that is conformed to the shape of theends. Each slot is preferably tapered such that its width narrows in adirection from the anterior end to the posterior end whereby the widthof the slot proximate the posterior end is less than the width of theend of the strut. The ends of the struts preferably have a lateral widththat tapers in substantially the same manner as the slots such that alocking taper engagement is formable between the slots and the ends ofthe struts.

The height of each strut preferably varies along the length of the strutsuch that the height between the engaging plates differs between theanterior end and the posterior end to allow the lordosis of the spine tobe maintained. The first and second struts may have differing heights tocause the height of the fusion device to vary along the device from thefirst side to the second side to correct for a lateral deviation in thespinal column. Each of the struts may include a hinge to allow an uppermember of the strut to pivot with respect to a lower member of thestrut.

In an alternate embodiment, the engaging plates include slots and thefusion device further includes a pair of pins disposed within the slots.Each engaging plate preferably includes a rib extending in asubstantially perpendicular direction from its face. The slot forreceiving the pins is preferably disposed on the rib. The pins arepreferably substantially elongated and may extend in a direction fromthe first side to the second side. The fusion device preferably furtherincludes a rotatable connector engaging the pins. Rotation of theconnector preferably causes movement of the pins relative to one anotherto alter the height of the fusion device to create a desired lordoticalignment.

The connector is preferably adapted to move axially between the engagingplates and may contain a retaining ring for contacting an engaging plateto limit movement of the connector through the fusion device. Theconnector preferably moves axially between the engaging plates in adirection from the anterior end to the posterior end, thereby moving thefirst pin toward the anterior end and the second pin toward theposterior end to increase the height between the engaging plates. Theconnector may be a screw having a threaded portion. The first pin mayinclude a threaded opening for receiving a threaded portion of theconnector. The second pin may be connected to an unthreaded portion ofthe connector.

The pins preferably include a receiving section and an end. The ends ofthe pins are preferably sized to fit within the slots in the ribs of theengaging plates. The receiving section may have a width greater thanthat of the ends of the pins and preferably contains an opening forreceiving the connector.

One engaging plate preferably includes a first slot that may terminatein an end that extends in a diverging direction from an end of anotherslot contained on the other engaging plate. Movement of one of the pinspreferably draws the ends of the slots together to alter the amount ofseparation between the engaging plates. The movement of the pinsrelative to one another preferably alters the height proximate theanterior end at a faster rate than the height proximate the posteriorend is altered to achieve a desired lordotic alignment.

In an alternate embodiment, the fusion device contains a load-sharingmember to promote a spinal fusion. The load-sharing member may beaxially disposed within the struts. The load-sharing member ispreferably substantially deflectable to allow movement of one of theengaging plates when a compressive force is exerted on the engagingplates. A predetermined spacing preferably exists between the upper andlower members. Application of a compressive force onto the engagingplates preferably deflects the load-sharing member and decreases thepredetermined spacing between the members, thereby decreasing the heightof the strut. The deflection of the load-sharing member preferablyimparts stress to bone graft proximate the engaging plates to promotethe development and growth of bone in accordance with Wolff's law.

The load-sharing member may be a pin having a circular cross-section andpreferably is disposed in a bore extending axially through the strut.The bore preferably has a greater width than that of the load-sharingmember to provide space for deflection of the load-sharing member. Theload-sharing member may serve as a hinge-pin about which the uppermember of the strut pivots with respect to the lower member of thestrut.

The fusion device preferably further includes a connector for engagingthe load-sharing member to impart force to the load-sharing member tocause it to deflect. The strut may include a threaded opening in its endfor receiving the connector. The predetermined spacing between the upperand lower members may be set to a desired length by altering theposition of the connector in the opening in the end of the strut. Theload-sharing member may include an indention having a substantiallyplanar surface to provide a site for engagement with the connector. Theconnector preferably engages the load-sharing member at a fulcrum pointlocated at a predetermined horizontal distance from a support locationwhere the lower member of the strut contacts the load-sharing member.The material properties of the load-sharing member and the distancebetween the fulcrum point and the support location are preferablycontrolled such that the modulus of elasticity across the strut issubstantially equal to the modulus of elasticity of bone.

The above embodiments may be used independently or in combination.

An advantage of the invention relates to an intervertebral body fusiondevice that substantially maintains the natural lordosis of the humanspine.

Another advantage of the invention relates to an intervertebral bodyfusion device adapted to correct a lateral deviation in the spinalcolumn.

Another advantage of the invention relates to an intervertebral bodyfusion device adapted to deflect to impart stress on surrounding bone topromote bone growth.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will become apparent tothose skilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 depicts a conventional intervertebral body fusion implantpositioned between neighboring vertebrae.

FIG. 2 depicts another conventional intervertebral body fusion implantthat includes a pair of cylindrical members positioned betweenneighboring vertebrae.

FIG. 3 depicts a top view of a fusion device located on a vertebralbody.

FIG. 4a depicts a cross-sectional view of the fusion device of FIG. 3taken along plane I.

FIG. 4b depicts a cross-sectional view of the fusion of FIG. 3 devicetaken along plane I wherein the fusion device contains bone graft andhas been adjusted to maintain a substantially natural lordosis.

FIG. 5 depicts a front view of a fusion device.

FIG. 6a depicts a perspective view of a strut.

FIG. 6b depicts a side view of a tapered strut.

FIG. 7 depicts a top view of a fusion device.

FIG. 8 depicts a front view of a pair of engaging plates.

FIG. 9 depicts a front view of a fusion device having pivotable struts.

FIG. 10 depicts a top view of a fusion device containing a connector.

FIG. 11 depicts an anterior view of a fusion device having a connectorand cam pins.

FIG. 12 depicts a cross-sectional view taken along plane m of FIG. 11 ofthe fusion device in a lowered position.

FIG. 13 depicts a cross-sectional view taken along plane III of FIG. 11of the fusion device in a raised position.

FIG. 14 depicts a cross-sectional view taken along plane IV of FIG. 11of the fusion device in a lowered position.

FIG. 15 depicts a cross-sectional view taken along plane IV of FIG. 11of the fusion device in a raised position.

FIG. 16 depicts a side view of a fusion device disposed betweenvertebrae.

FIG. 17 depicts a top view of a strut having a tapered end.

FIG. 18 depicts a cross-sectional view taken along plane V of FIG. 17 ofthe strut in an unloaded position.

FIG. 19 depicts a cross-sectional view taken along plane V of FIG. 17 ofthe strut in a loaded position.

FIG. 20 depicts a top view of a fusion device located on a vertebralbody.

FIG. 21 depicts a cross-sectional view of the fusion device taken alongplane VI of FIG. 3.

FIG. 22 depicts a top view of a conventional fusion cage having a pairof cylindrical elements disposed on a vertebra.

FIG. 23 depicts a side view of one of the cylindrical elements in FIG.22 disposed between neighboring vertebrae.

FIG. 24 depicts a front view of the cylindrical element in FIG. 23.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of an interbody fusion implant device 10 forfacilitating the formation of a spinal fusion is depicted in FIGS. 3-5.A top view of the fusion device is depicted in FIG. 3. Fusion device 10preferably includes a pair of sides or engaging plates 12 and 14 forengaging vertebral bodies 16 and 18. The engaging plates may containcurved edges such that the outer face 15 of the plates conforms to theshape of the cross-section of the vertebral bodies as shown in FIG. 3.The fusion device has a height 20 defined by the vertical distancebetween the outer faces 15 of the engaging plates 12 and 14. The height20 of the fusion device is preferably adjustable and may vary along thefusion device between anterior end 22 and posterior end 24 to maintainthe natural lordosis of the spine. Height 20 may also vary along device10 from first side 26 to second side 28 to correct for a lateraldeviation in the spine as may occur in scoliosis. Fusion device 10preferably further includes an alignment device for adjusting the height20 so that the natural lordosis of the spine is substantially maintainedafter the fusion device is implanted. The alignment device may be usedto adjust the height between the engaging plates proximate the anteriorend and independently adjust the height between the engaging platesproximate the posterior end.

A spinal fusion is typically employed to eliminate pain caused by themotion of degenerative disk material. Upon successful fusion, fusiondevice 10 becomes permanently fixed within the disc space. The fusiondevice is preferably packed with bone graft 40 to promote the growth ofbone through and around the fusion device. Such bone graft may be packedbetween engaging plates 12 and 14 prior to, subsequent to, or duringimplantation of the fusion device. Bone substitute material that is wellknown to those skilled in the art may be used instead of bone graft. Abone harvester kit, commercially available from Spine-Tech, Inc. locatedin Minneapolis, Minn., may be used to inject bone graft between theengaging plates. The pamphlet entitled “Bone Harvester: MinimallyInvasive Bone Harvesting Kit” (available from Spine-Tech, Inc.) detailsthe use of the bone harvesting kit.

In an embodiment of the invention, the faces 15 of engaging plates 12and 14 contain a plurality of openings 34 disposed therein to allow bonedevelopment and growth through the engaging plates 12 and 14 and betweenfusion device 10 and neighboring vertebrae 16 and 18. In an embodiment,the openings 34 have a combined area that is greater than about 50percent of the area of face 15 (including the area of the openings 34),more preferably between about 60 percent and about 80 percent of thearea of face 15, and more preferably still about 70 percent or more ofthe area of face 15.

The fusion device may contain a retaining plate 36 proximate posteriorend 24 to provide a backing against which bone graft may be packed andto maintain the bone graft between the engaging plates. Retaining plate36 may be substantially planar and may contain openings to allow boneingrowth therethrough. A removable endcap 25 may be positioned proximateanterior end 22 to contain bone graft within the fusion device and toprevent the migration of bone graft outside the engaging plates. Theendcap 25 may contain one or more openings for allowing bone ingrowthbetween a vertebral body and bone graft contained between the engagingplates. Endcap 25 is preferably made of a plastic material such aspolyethylene that tends to be non-irritating and non-abrasive to thesurrounding tissues.

A cross section of the fusion device taken through plane I of FIG. 3 isdepicted in FIG. 4a and FIG. 4b. FIG. 4a shows the relative position ofengaging plates 12 and 14 before height 20 has been adjusted with analignment device to achieve a substantially natural lordosis. FIG. 4bshows the relative position of the plates after height 20 has beenadjusted and bone graft 40 has been packed between the engaging plates.FIG. 4b shows that height 20 is greater in the vicinity of anterior end22 as compared to posterior end 24 to maintain the natural lordosis ofthe spinal column. The faces 15 of the engaging plates 12 and 14 arepreferably planar to provide a relatively large contact area between theengaging plates and the neighboring vertebrae. In this manner,subsidence of the vertebrae may be prevented because the force impartedto the vertebrae from the fusion device is not concentrated across arelatively small area of the vertebrae as in some conventional implants.Alternately, the engaging plates may be non-planar. The engaging platesalso preferably contain a plurality of spikes or protrusions 38extending from the face 15 for enhancing an engagement between thevertebra and the engaging plate. The protrusions may extend into thevertebra to prevent the fusion device from moving out of the disc space.The engaging plates are preferably constructed of titanium or a titaniumalloy, although it is to be understood that other materials (e.g.,ceramics, metals, carbon composites) may be used.

A front view of the fusion implant device is depicted in FIG. 5. In anembodiment of the invention, the alignment device includes a first strut30 and a second strut 32 that each extend between engaging plates 12 and14 along the length of the fusion device from anterior end 22 toposterior end 24. As described herein, a “strut” is taken to mean anysupport member disposed between the engaging plates to separate theengaging plates. Strut 30 preferably extends along the fusion deviceproximate first side 26. Strut 32 is preferably substantially parallelto strut 30 and may extend along the fusion device proximate second side28. The struts 30 and 32 serve to create a predetermined spacing betweenthe engaging plates. The predetermined spacing is preferably such thatthe height 20 is approximately equal to the height of the disc materialthat formerly occupied the disc space between the vertebral bodies.

A perspective view of an embodiment of the strut is depicted in FIG. 6a.The strut may have an “I-beam” shape and preferably includes a pair ofends 50. The ends 50 may have a lateral width 51 that is greater thanthat of the sides 53. The ends preferably have a “dovetail” shapedcross-section as shown in FIG. 6a. The engaging plates preferablycontain elongated slots 60 (shown in FIGS. 7 and 8) sized to receiveends 50 of the first and second struts. The slots 60 preferably have acomplementary dovetail shape as depicted in FIG. 8 that conforms to theshape of the end 50. The struts may be connected to the engaging platesby sliding ends 50 into the slots 60 in a direction from anterior end 22to posterior end 24 or vice versa.

In an embodiment, the slots are tapered such that their width narrows ina direction from the anterior end to the posterior end as shown in FIG.7. The ends 50 may be tapered (as shown in FIG. 17) such that thelateral width 51 narrows along the length of the strut. The taper of thelateral width of the strut preferably matches that of slot 60. The widthof the slot proximate the anterior end is preferably sized to allow thestrut end to be slid into the slot. The width of the slot proximate theposterior end is preferably less than the lateral width 51 of thenarrowest portion of end 50. The tapering of the slots preferably allowsa “locking taper engagement” of the strut ends within the slots. A“locking taper engagement” is taken to mean a fixable interference fitformed between end 50 and slot 60 whereby the strut resists dislodgementwhen force is imparted to the fusion device from the adjacent vertebrae.In an alternate embodiment, the slots may be tapered such that the widthof the slots narrows in a direction from the posterior end to theanterior end.

The first and second struts preferably each have a predetermined heightthat defines the height of the fusion device. The engaging plates 12 and14 are preferably adapted to receive struts of various heights to allowheight 20 to be varied to fit the needs of the patient. A side view of atapered strut is depicted in FIG. 6b. The tapered strut preferably has aheight that varies along its length. In this manner, the tapered strutis positionable between the engaging plates 12 and 14 to cause height 20to decrease in a direction from anterior end 22 to posterior end 24whereby the natural lordosis of the human spine is maintained by thefusion device. The degree of taper of the strut corresponds to a desiredlordosis and may vary depending upon the size of the patient.

In an embodiment, the first and second struts have differing heights tocause height 20 to vary between first end 14 and second end 16. In thismanner, the fusion device may be used to correct a lateral deviation inthe spinal column as may occur in scoliosis. A front view of a fusiondevice containing struts having different heights is depicted in FIG. 9.Each of the struts preferably contains a hinge pin 70 to allow an uppermember 72 of the strut to pivot with respect to a lower member 74 of thestrut. In this manner, the struts may be pivoted as shown in FIG. 9 suchthat the ends of the struts are properly aligned with the slots of theengaging plates when a height difference exists between the first andsecond struts.

To install the fusion device, a discectomy is preferably performed froman anterior approach. All cartilage and soft tissue are preferablyremoved from the vertebral endplate as would normally be done forplacement of a femoral strut graft. Such a procedure is well within theknowledge of a skilled practitioner of the art. The engaging plates maybe deployed in the disc space between the adjacent vertebrae. Adistraction force may be applied to the engaging plates using a laminaespreader or similar device to force the vertebrae to a selected heightand lordotic alignment. The use of a laminae spreader is well known tothose skilled in the art. The proper heights for the first and secondstruts may be determined beforehand using x-ray techniques in which theposterior and anterior portions of the intervertebral disc space areexamined.

Appropriately sized and tapered struts are preferably slipped into slots60 and tapped until a locking taper engagement is achieved between thestrut ends and the slots. If struts of differing heights are used tocorrect for a lateral deviation in the spinal column, each strut may bepivoted about hinge pin 70 prior to insertion so that ends 50 areproperly aligned for placement into grooves 60. Bone graft material ispreferably inserted through the anterior end and packed between theengaging plates. Retaining plate 36 preferably prevents the bone graftmaterial from passing through the fusion device during packing. Endcap25 may then be placed onto the anterior end.

In an alternate embodiment depicted in FIGS. 10-16, the alignment deviceincludes a connector 80 for adjusting the height 20 of the plates toachieve a desired lordotic alignment. FIG. 10 depicts a top view of thefusion device. Connector 80 is preferably a drive screw that isrotatable to adjust height 20. Connector 80 preferably extends betweenengaging plates 12 and 14 and may be adapted to move axially through thefusion device in a direction from anterior end 22 to posterior end 24.The engaging plates may contain elongated openings 82 for allowing bonegrowth through the faces 15 of the plates.

FIG. 11 depicts a front (anterior) view of the fusion device in a raisedposition. In an embodiment, the engaging plates include ribs 84 and 85that may extend substantially perpendicularly from face 15. Across-sectional view taken along plane III of FIG. 11 is depicted ineach of FIG. 12 and FIG. 13. FIG. 12 depicts rib 84 and cam pins 86 and88 in section with the fusion device in a “lowered position” (i.e.,unadjusted for lordotic alignment). FIG. 13 depicts the rib and cam pinsin section with the fusion device in the “raised position” (i.e.,adjusted for lordotic alignment). As described herein, “cam pin” istaken to mean any connecting element capable of extending from theconnector into the slots 90 and 92. Each of the cam pins may beintersected by an imaginary longitudinal axis 91 axially extendingthrough the fusion device.

Rib 84 preferably contains a slot 90 having a first end and a secondend. The ends of slot 90 preferably terminate in a direction below axis91. The first end of slot 90 preferably extends downwardly substantiallytoward either the face of engaging plate 14 or the anterior end. Thesecond end of slot 90 preferably extends downwardly substantially towardeither the face of engaging plate 14 or the posterior end. Rib 85preferably contains a slot 92 having a pair of ends that extend indiverging directions from the slot ends of rib 84. The ends of slot 92preferably terminate in a direction above axis 91. The first end of slot92 preferably extends upwardly substantially toward either the face ofengaging plate 12 or the anterior end. The second end of slot 90preferably extends upwardly substantially toward either the face ofengaging plate 12 or the posterior end. The engaging plates arepreferably connected together with cam pins 86 and 88, which preferablyhave ends sized to fit within slots 90 and 92. The cam pins preferablyare disposed through the fusion device in a direction from the firstside to the second side. Pins 86 and 88 preferably contain a receivingsection 87 having an opening for receiving connector 80. Receivingsection 87 may have a greater width (e.g., diameter) than the ends ofpins 86 and 88 disposed in slots 90 and 92.

FIG. 14 and FIG. 15 each depict a cross-sectional view of the fusiondevice taken along plane IV of FIG. 11. FIG. 14 depicts the connectorand cam pins in section with the fusion device in the lowered position.FIG. 15 depicts the connector and the cam pins in section with thefusion device in the raised position. In an embodiment, connector 80contains a threaded portion 94 and an unthreaded portion 96. Pin 86 ispreferably connected to the threaded portion and pin 88 is preferablyconnected to the unthreaded portion.

In an embodiment, a torque delivered to the connector is converted intoa separation force between the cam pins. Rotating the connector in acounterclockwise direction preferably moves the connector in a directionfrom the anterior end to the posterior end. Pin 88 is preferablyattached to the connector and preferably moves in the same manner as theconnector. Pin 86 preferably contains an opening having complementarythreading to that of the connector. Pin 86 preferably moves toward theanterior end in a direction opposite the motion of the connector toincrease the separation between pin 88 and pin 86. The ends of the pinspreferably move along the angled portions of the slots 90 and 92,causing the ends of the slots to be drawn together. In this manner, theseparation between the engaging plates is increased. The connector maybe rotated in a clockwise direction to move the connector in a directionfrom the posterior end to the anterior end, thereby decreasing height20.

Conventional methods of surgically implanting fusion devices tend torequire that distraction instruments be inserted between the vertebraeto separate them and allow insertion of the fusion device therebetween.The surgical incision typically must be widened to accommodate thedistraction instruments. In an embodiment, the fusion device in thelowered position has a height that is less than the disc space betweenthe vertebrae. In this manner, the fusion device may be inserted betweenthe vertebrae with minimal distraction. Connector 80 is preferablyoperable to separate the engaging plates (hence the vertebrae) andcreate a desired lordotic alignment.

The distance that the engaging plates are separated per unit torqueapplied to the connector will tend to depend upon the angle of the slots90 and 92. The slots are preferably angled such that the height 20proximate the anterior end changes at a greater rate than the height 20proximate the posterior end when the connector is adjusted to alter thedistance between the plates. In this manner, a desired lordoticalignment may be achieved. It is to be understood that the fusion deviceis operable in a semi-raised position that is intermediate the raisedand lowered positions depicted in FIGS. 12-15. The connector ispreferably rotated to a selected degree to achieve a preferred height 20proximate the anterior and posterior ends to suit the particularpatient. The angle of the slots 90 and 92 may vary among patients and ispreferably selected to achieve a desired lordotic alignment. Theconnector may include a retaining ring 98 for contacting one or both ofthe engaging plates to limit the degree to which the connector can movethrough the fusion device.

FIG. 16 depicts a side view of an alternate embodiment of the fusiondevice installed between neighboring vertebrae. Pin 86 may be located onthe unthreaded portion of the shank adjacent to the head of connector80. Pin 88 may be located on threaded portion 94 of the shank ofconnector 80. Rib 84 preferably includes a first slot 100 that is angleddiagonally upward from axis 91 in a direction substantially towardeither the face of engaging plate 12 or the anterior end 22. Rib 84preferably also includes a second slot: 102 that is angled diagonallyupward from axis 91 in a direction substantially toward either the faceof engaging plate 12 or the posterior end 24. Rib 85 preferably includesa first slot 104 that is angled diagonally downward from axis 91 in adirection substantially toward either the face of engaging plate 14 orthe anterior end 22. Rib 85 preferably also includes a second slot 106that is angled diagonally downward from axis 91 in a directionsubstantially toward. either the face of engaging plate 14 or theposterior end 24. To adjust the fusion device into the raised position,the connector may be rotated to cause the cam pins to be moved in adirection toward one another. Pin 86 preferably moves with the connectorin a direction from the anterior end to the posterior end to increasethe separation between the engaging plates proximate the anterior end.Pin 88 preferably contains a threaded opening for receiving theconnector and may move in a direction toward the posterior end toincrease the separation between the engaging plates proximate theposterior end.

In an alternate embodiment, each of the pins 86 and 88 contains athreaded opening for receiving the connector 80. The connector may be a“double-threaded” screw having two threaded portions for complementingthe threaded openings of the pins 86 and 88. Rotation of the screw in afirst direction preferably causes the pins to move toward one another toincrease the separation between the engaging plates. Rotation of thescrew in an opposite direction preferably causes the pins to move awayfrom one another to reduce the separation between the engaging plates.

In an alternate embodiment, the alignment device includes a load-sharingmember to allow the engaging plates to move in response to a compressiveforce of predetermined magnitude. In accordance with Wolff's law, bonegrowth tends to occur in the presence of stress (e.g., load), and bonetends to be absorbed in the absence of stress. The load-sharing memberpreferably enables the fusion device to “share” compressive forcesexerted onto the spinal column with the bone graft in the vicinity ofthe fusion device. The load-sharing member preferably is deflected uponreceiving a predetermined force to cause the engaging plates to move,thereby shifting load from the fusion device to the bone graft proximatethe fusion device. It is believed that providing a selected amount ofstress to the bone graft in a such a manner will tend to result in ahigher fusion rate as well as a stronger fusion mass.

An embodiment of the load-sharing fusion device is depicted in FIGS.17-19. A top view of a strut 30 containing a load-sharing member isdepicted in FIG. 17. FIGS. 18 and 19 depict cross-sectional views of thestrut taken along plane V of FIG. 17. Load-sharing member 110 ispreferably disposed axially through the strut. The load-sharing membermay be contained in a bore extending into the strut. The bore preferablyhas a width (e.g., diameter) that is greater than that of theload-sharing member to allow sufficient space for the load-sharingmember to be deflected. The bore is preferably disposed within lowermember 74. Portion 118 of the upper member may substantially surroundthe bore and the load-sharing member, thereby allowing attachment of theupper and lower members. In an embodiment, the load-sharing member is apin having a substantially circular cross-section. The pin preferablyfits loosely within the bore such that its rotational freedom ismaintained. The pin may be hinge pin 70 about which the upper member 72pivots with respect to the lower member 74. The load-sharing memberpreferably contains an indention 114 forming a substantially planarsurface about which the load-sharing member may be deflected.

A connector 112 preferably extends through an opening 116 in the end 50of the strut. The connector preferably fixes the load-sharing member tothe upper member 72 and may contact the load-sharing member at fulcrumpoint 126, which is preferably located on the planar surface formed byindention 114. Connector 122 is preferably a set screw, and opening 116preferably contains threading for engaging the set screw. FIG. 18depicts the strut in an “unloaded” position whereby a predeterminedspacing 122 exists between upper member 72 and portion 120 of lowermember 74. The predetermined spacing 122 may be adjusted by altering thelocation of connector 112 within opening 116. For instance, the screwmay be rotated through opening 116 to increase spacing 122. Theload-sharing member preferably remains substantially undeflected in theunloaded position.

Upon application of a compressive force onto the end 50 of the uppermember 72, force is preferably imparted from connector 112 to theload-sharing member at fulcrum point 126. The compressive force ispreferably sufficient to cause deflection of the load-sharing member andmovement of upper member 72 toward portion 120 of the lower member suchthat predetermined spacing 122 is decreased. The deflection of theload-sharing member may force portion 118 of the upper member into acavity 115 formed within the axial bore. The load-sharing member ispreferably deflected in a three point bending arrangement as shown inFIG. 19.

FIG. 19 depicts the strut in the “loaded” position with the load-sharingmember deflected. The predetermined spacing 22 is preferably adjustableand may be adjusted to set the maximum strain that can be imparted tothe load-sharing member. When the load-sharing member has been deflecteda vertical distance equal to predetermined spacing 22, the upper member72 contacts portion 120, thereby inhibiting further strain on theload-sharing member. In this manner, the maximum amount of strain on theload-sharing member can be limited to reduce the possibility that themember will experience fatigue failure.

The load-sharing member may be constructed of any of a variety of metalsor alloys. In a preferred embodiment, the load-sharing member isconstructed of titanium or a titanium alloy. The material properties andcross-sectional area of the load-sharing member are preferablycontrolled to allow a predetermined amount of stress to occur across thefusion device. The horizontal distance 124 or moment arm between fulcrumpoint 126 and support point 128 on the lower member is preferablyselected such that the fusion device has an “effective” modulus ofelasticity in the vicinity of the modulus of elasticity of bone tofacilitate bone development. The “effective” modulus of elasticity ofthe fusion device is taken to mean the ratio of stress to strain acrossthe fusion device in a direction along height 20 as the device movesfrom the unloaded position to the loaded position upon receiving acompressive force. As described herein, “in the vicinity of the modulusof elasticity of bone” is taken to mean a Young's modulus between about3 GPa and about 25 GPa. In an embodiment, the effective modulus of thefusion device is between about 16 GPa and about 20 GPa. The paperentitled “Variation of Young's Modulus and Hardness in Human LumbarVertebrae Measured by Nanoindentation” by Marcel Roy and Jae-Young Rho(Department of Biomedical Engineering, University of Memphis, Memphis,Tenn.), and Ting Y. Tsui and George M. Pharr (Department of MaterialsScience, Rice University, Houston, Tex.) relates to the mechanicalproperties of bone and is incorporated by reference as if fully setforth herein.

The stresses exerted onto the spinal column are preferably shared by thefusion device and surrounding bone graft. As the spinal fusion develops,the proportion of stress experienced by the surrounding bone materialpreferably increases and the required load on the fusion devicepreferably decreases. After completion of the fusion, the fusion devicepreferably remains in the unloaded position during normal daily activityof the patient.

Fusion device 10 preferably provides a relatively large contact areabetween the engaging plates and the vertebral bodies defining the discspace occupied by the fusion device. FIG. 20 depicts a top view of anembodiment of a fusion device of the present invention. FIG. 21 depict across-sectional view of the fusion device taken along plane VI of FIG.20. Depicted in FIGS. 22-24 is a conventional fusion cage such as thatdescribed in U.S. Pat. No. 4,961,740 to Ray et al. This patent isincorporated by reference as if fully set forth herein. The devices inFIGS. 20-24 are sized for use in the L3-L4 disc space of an average sizemiddle-aged female. Dimensions of the fusion devices are indicated inmillimeters.

The “effective contact area” between an engaging plate and a vertebralbody may be calculated by subtracting the fenestration area, a (i.e.,the combined area of the openings 34 intended for bone ingrowth), fromthe total contact area, A (the area of the face 15 including the area ofthe openings 34). The total contact area and the fenestration area ofthe fusion device in FIGS. 20 and 21 is 581 mm² and 96 mm²,respectively. Therefore, the effective contact area between the engagingplate and the vertebra is 485 mm².

For the fusion cage depicted in FIGS. 22-24, it is assumed that threadson the outer surface of the fusion cage penetrate into the vertebra atotal of 3 mm per side as recommended by the manufacturer. It should benoted that such penetration is often difficult to achieve. In addition,the cortical layer of a vertebral body is often only 1-2 thick. Each ofthe cylindrical elements of the fusion cage has a total contact area of283.5 mm² and a fenestration area of 198.5 mm². Therefore, the combinedeffective contact area of both of the cylindrical elements is 170 mm².If the threads of the fusion cage penetrate into the vertebra a distanceless than 3 mm per side, the contact area will be less than thatcalculated above.

The maximum axial compressive forces in the lumbar spine resulting fromeveryday activity were estimated to be 3200 N in a paper entitled “TheBAK™ Interbody Fusion: An Innovative Solution” by Bagby et al. andavailable from Spine Tech, Inc. in Minneapolis Minn. (see page 3, bottomparagraph). For a 3200 N compressive force, the stress per unit area iscalculated to be 18.8 N/mm² for the fusion cage depicted in FIGS. 22-24as compared to 6.6 N/mm² for the fusion device depicted in FIG. 20 andFIG. 21. It is believed that such a reduction in stress per unit areawill result in a significant reduction in post surgical subsidence atthe interface of the fusion device and vertebral body. Typically, theloss of disc height is estimated to be about 1-3 mm at one monthfollow-up when conventional devices such as that depicted in FIGS. 22-24are employed.

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

What is claimed is:
 1. A spinal implant for facilitating a fusionbetween neighboring vertebrae of a human spine, comprising: a pair ofengaging plates adapted to fit between and engage the vertebrae tomaintain a disc space between the vertebrae during use; and an alignmentdevice configured to be positioned between the engaging plates, whereinthe alignment device is adapted to separate anterior and posterior endsof the engaging plates during use such that a cavity is formed, andwherein the alignment device is configured to compress during use; andwherein a height between outer surfaces of the pair of engaging platesproximate anterior ends of the engaging plates differs from a heightbetween outer surfaces of the pair of engaging plates proximateposterior ends of the engaging plates when the alignment device ispositioned between the engaging plates during use.
 2. The spinal implantof claim 1, wherein differing height between outer surfaces of the pairof engaging. plates proximate anterior ends of the engaging plates andouter surfaces of the pair of engaging plates proximate posterior endsof the engaging plates is configured to align the pair of engagingplates such that the engaging plates have a lordotic alignment.
 3. Thespinal implant of claim 1, wherein the height between the outer surfacesof the pair of engaging plates proximate the anterior ends of theengaging plates is greater than the height between the outer surfaces ofthe pair of engaging plates proximate the posterior ends of the engagingplates when the alignment device is positioned between the engagingplates.
 4. The spinal implant of claim 1, wherein a first engaging plateof the pair of engaging plates further comprises a protrusion, andwherein the protrusion is configured to extend into a vertebra of theneighboring vertebrae.
 5. The spinal implant of claim 1, wherein a firstengaging plate of the pair of engaging plates further comprises atapered slot.
 6. The spinal implant of claim 5, wherein the alignmentdevice comprises a tapered surface configured to form a lockingengagement with the tapered slot.
 7. The spinal implant of claim 1,wherein a surface of a first engaging plate of the pair of engagingplates is substantially planar to inhibit subsidence of a vertebra ofthe neighboring vertebrae.
 8. The spinal implant of claim 1, wherein afirst engaging plate of the pair of engaging plates comprises aplurality of openings configured to allow bone growth to occur thatfuses the implant and the neighboring vertebrae during use.
 9. Thespinal implant of claim 8, wherein an area of the plurality of openingsis greater than about 50 percent of a total outer surface area of thefirst engaging plate.
 10. The spinal implant of claim 1, wherein a firstengaging plate of the pair of engaging plates comprises an openingconfigured to allow bone growth to occur that fuses the implant and theneighboring vertebrae.
 11. The spinal implant of claim 1, wherein thealignment device comprises a first strut and a second strut.
 12. Thespinal implant of claim 1, further comprising a substantially planarretaining plate configured to be positioned between the engaging plates.13. A spinal implant for facilitating fusion of vertebrae, comprising: afirst engaging plate, the first engaging plate having a surfaceconfigured to couple to a first vertebra, wherein the first engagingplate comprises a plurality of openings configured to allow bone growthto occur that fuses the spinal implant and the first vertebrae; analignment device coupled to the first engaging plate, wherein thealignment device is configured to compress; a second engaging plate, thesecond engaging plate having a surface configured to couple to a secondvertebra, and wherein the second engaging plate is configured to becoupled to the alignment device; and wherein the alignment device isadapted to separate anterior and posterior ends of the engaging platesduring use, and wherein a height between the surface of the firstengaging plate and the surface of the second engaging plate proximate ananterior end of the spinal implant differs from a height between thesurface of the first engaging plate and the surface of the secondengaging plate proximate a posterior end of the spinal implant.
 14. Thespinal implant of claim 13, wherein a difference in height between thesurface of the first engaging plate and the surface of the secondengaging plate proximate an anterior end of the spinal implant differsfrom a height between the surface of the first engaging plate and thesurface of the second engaging plate proximate a posterior end of thespinal implant is adapted to provide lordotic adjustment.
 15. The spinalimplant of claim 13, wherein the height proximate the anterior end ofthe spinal implant is greater than the height proximate the posteriorend of the spinal implant.
 16. The spinal implant of claim 13, whereinthe first engaging plate comprises a protrusion, and wherein theprotrusion is configured to extend into a vertebra of the neighboringvertebrae.
 17. The spinal implant of claim 13, wherein the firstengaging plate further comprises a tapered slot.
 18. The spinal implantof claim 17, wherein the alignment device comprises a tapered surfaceconfigured to form a locking engagement with the tapered slot.
 19. Thespinal implant of claim 13, wherein the surface of the first engagingplate is substantially planar to inhibit subsidence of the firstvertebra.
 20. The spinal implant of claim 13, wherein an area of theplurality of openings is greater than about 50 percent of a totalsurface area of the surface of the first engaging plate.
 21. The spinalimplant of claim 13, wherein the first engaging plate comprises anopening configured to allow bone growth to occur that fuses the spinalimplant and first vertebra.
 22. The spinal implant of claim 13, whereinthe alignment device comprises a first strut and a second strut.
 23. Thespinal implant of claim 22, wherein the first strut forms a lockingengagement with the first engaging plate during use.
 24. The spinalimplant of claim 22, wherein the second strut forms a locking engagementwith the second engaging plate during use.
 25. The spinal implant ofclaim 13, further comprising a substantially planar retaining plateconfigured to be positioned between the first engaging plate and thesecond engaging plate.
 26. A fusion implant, comprising: a pair ofengaging plates adapted to fit between and engage a pair of vertebraeduring use, wherein a first engaging plate comprises a plurality ofopenings configured to allow bone growth to occur that fuses the spinalimplant and the first vertebrae; and an alignment device configured tobe positioned between the engaging plates, the alignment device adaptedto separate anterior and posterior ends of the engaging plates duringuse, and wherein the alignment device is configured to compress.
 27. Thefusion implant of claim 26, wherein a height between outer surfaces ofthe pair of engaging plates proximate an anterior end of the fusionimplant differs from a height between outer surfaces of the engagingplates proximate a posterior end of the fusion implant.
 28. The fusionimplant of claim 26, wherein a difference in the height between outersurfaces of the pair of engaging plates proximate an anterior end of thefusion implant and the height between outer surfaces of the engagingplates proximate a posterior end of the fusion implant provide lordoticadjustment.
 29. The fusion implant of claim 27, wherein the heightbetween outer surfaces of the pair of engaging plates proximate theanterior end is greater than the height between outer surfaces of thepair of engaging plates proximate the posterior end.
 30. The fusionimplant of claim 26, wherein a first engaging plate of the pair ofengaging plates comprises a protrusion configured to couple the firstengaging plate to a vertebra of the pair of vertebrae.
 31. The fusionimplant of claim 26, wherein a first engaging plate of the pair ofengaging plates comprises an opening configured to allow bone growththat fuses the fusion implant and the vertebrae.
 32. The fusion implantof claim 26, wherein an area of the openings is greater than about 50percent of an upper surface area of the first engaging plate.
 33. Thefusion implant of claim 26, wherein a first engaging plate of the pairof engaging plates further comprises a tapered slot.
 34. The fusionimplant of claim 26, wherein the alignment device comprises a taperedsurface configured to form a locking engagement with the tapered slot.35. The fusion implant of claim 26, wherein a surface of a firstengaging plate of the pair of engaging plates is substantially planar toinhibit subsidence of a vertebra of the neighboring vertebrae.
 36. Thefusion implant of claim 26, wherein the alignment device comprises afirst strut and a second strut.
 37. The fusion implant of claim 36,wherein a height of the first strut differs from a height of the secondstrut so that a height of a first side of the fusion implant differsfrom a height of a second side of the fusion implant when the fusionimplant is assembled.
 38. The fusion implant of claim 36, wherein thefirst strut and the second strut form locking engagements with theengaging plates.
 39. The fusion implant of claim 26, further comprisinga substantially planar retaining plate configured to be positionedbetween the engaging plates.
 40. The spinal implant of claim 11, whereinthe first strut and the second strut form locking engagements with theengaging plates.